In storage (recording) devices used for data and information systems, semiconductor memories and magnetic memories are generally used for data storage. For applications specifying short access times, semiconductor memories are generally used in internal storage devices. For applications specifying huge capacity and non-volatility, hard disk drives (HDDs) are generally used which employ magnetic disks as external storage devices. Storage capacity is an important index to indicate the capability of magnetic disk devices. Magnetic disk devices with huge capacity and having compact size have been increasingly requested by the market with recent developments of the information age.
The volume of information processing in the information age is increasing rapidly. In particular, HDDs, which are external storage devices, have been desired to store more information in its limited area and volume. A technical approach to this desire is to increase the capacity by increasing the recording density of the HDD. To achieve higher recording density, further miniaturization of recording bits is effective. Hence, in this approach, the size of a particle which is the unit of recording in a medium has been reduced.
The size of magnetic heads have been progressively reduced year after year in order to increase the recording density of a HDD using such heads. While an induction-type head combining a write head with a read head was mainly used in the past, a write/read, separated type head is currently being used generally for improving the performance of the HDD. While a write head is an induction head that writes information by using an induction magnetic field generated by a coil, a GMR (Giant Magneto-Resistance) head or TMR (Tunneling Magneto-Resistance) head using a spin valve as a magnetic sensor is used for a read head. FIG. 2 shows a schematic view of a write/read separated type head having a write head section 1 and a read head section 2. FIG. 3 shows an enlarged view of the read head section. FIG. 3 is a view seen from a surface facing a magnetic disk medium (ABS surface, and a surface seen from A in FIG. 2). The magnetic head is formed using fine processing technology on an Al2O3—TiC (alumina-titanium carbide, hereinafter referred to as AlTiC) substrate. In respective figures including figures shown later, a substrate portion is omitted from the views for the sake of simplicity. A lower magnetic shield layer 3 and an upper magnetic shield layer 4, each including permalloy, serve as electrodes, which apply a current to a multilayer-film spin-valve-type magneto-resistive-effect film 5 to detect a magnetic field. Alumina insulating film 11 is used to isolate a portion of the magneto-resistive-effect film 5 that detects the magnetic field. In addition, a permanent magnetic film 6 applies a bias magnetic field and is disposed adjacent to the magneto-resistive-effect film 5.
Changes to the track width Tw and gap length Gs can be helpful from a viewpoint of increasing an areal recording density of information for a read head. The respective dimensions correspond to resolution in size in a radial direction (track width) and resolution in size in a circumferential direction (bit length) of a recording bit on a magnetic disk. Drastic increase in areal recording density up to the annual rate of 40% has been attained through reducing these dimensions. However, gap length Gs approximately reaches a technological limit of size reduction unlike the track width Tw. The reason for this is that while the track width Tw is to be obtained by processing a magneto-resistive-effect film and therefore may be reduced with improvement in fine patterning using advanced lithography technology, the gap length Gs has a physical limit that corresponds to a thickness large enough to accommodate at least a magnetic sensor. In currently mass-produced products, Tw from about 60 nm to 90 nm and Gs from about 30 nm to 40 nm are given as typical dimensions of the track width Tw and the gap length Gs, respectively. Either GMR or TMR heads are regarded to have a thickness of at least 25 nm to act as a spin valve, and therefore any method which can achieve a head which has a gap length Gs of 25 nm or less would be very beneficial.
A differential-type head is known to those of skill in the art as a technology that may achieve an ultra-narrow Gs head. FIG. 4 shows a structure and an equivalent circuit of a differential-type head. Two spin valves 15 and 16 are stacked as a read sensor, and a sense current 17 is flowed through the spin valves 15 and 16 so as to obtain a differential output between the two elements, so that a change in an external magnetic field can be detected. Resolution in a Gs direction corresponding to a recording bit length depends on a distance Gs between the shields in previous single element-type heads. However, the resolution of a differential-type head depends on a distance GL between free layers as magnetic detection films of the two elements. This leads to a possibility where GL can be reduced to a lower limit of Gs or less, so that the resolution of the head is improved. A technology using a differential-type read head is disclosed in Japanese Patent Office Pub. No. 3760095, for example.
While the possibility of improving resolution in the bit direction by a differential-type read head structure for HDD is previously known, the possibility has not been applied in practice because a manufacturing process of the head is too difficult to perform consistently. Moreover, even a previous single-element structured head has so far been able to meet the technical objectives by reducing thickness of a single-layer spin valve. However, such a reduction in thickness will soon reach the described technical limit. In addition, the following problems in a manufacturing process of the head exist currently.
A first problem of current techniques is that film characteristics of an upper element deposited later tend to be degraded, which is due to continuous deposition of the two elements. It has been reasoned that this is due to crystal growth of a previously deposited lower element which causes an increase in a roughness of a surface to be a foundation for the upper layer. A second problem of current techniques is that element characteristics tend to be degraded during an ion milling step for processing a sensor film including the deposited, two elements to have a desired track width. In the differential-type structure, since total thickness of the two elements, namely, thickness two times as large as previous thickness is subjected to processing, there is a high probability that element characteristics are degraded due to damage to the end face of the element exposed during processing, or due to redeposit caused by ion milling.
Therefore, a method of producing a differential-type head which avoids the problems currently associated with differential-type head construction would be greatly beneficial to reducing bit size and increasing areal density of HDDs.